This invention relates to a beam compression method for radar antenna patterns for removing any distortions in the synthetic reception pattern generated when performing beam compression on an antenna pattern based on the principle of multiplicative array, thereby achieving an improvement in terms of beam compression performance.
Generally, beam width is one of the indexes indicating the quality of the antenna pattern of a receiving antenna, etc. The smaller the beam width, the better the quality of the antenna pattern. However, the beam width and the size (length) of an antenna are inversely proportional to each other. Thus, an attempt to reduce the beam width will result in an increase in the antenna size. On the other hand, reducing the antenna size leads to an increase in the beam width.
For example, in a radar antenna, doubling the power of identification, i.e., the resolution, involves a reduction in the beam width by half, resulting in the antenna size being doubled. Such an increase in antenna size not only results in an increase in the area occupied by the antenna but so leads to various problems, such as an increase in the weight of the antenna and the size of the structure supporting it. On the other hand reducing the antenna size by half results in the beam width being doubled, resulting the identification power being reduced by half.
This reciprocal relationship between beam width and antenna size is well known. In most antennas in use, there is a limit to the area they can occupy, etc. Thus, a compromise has to be made with a certain measure of beam width.
To solve this problem, a beam compression method has been known in which received signals of a plurality of antennas are multiplied in accordance with the principle of multiplication array so as to attain to a reduction in beam width. FIG. 1 shows an antenna device for effecting such a beam compression. In the drawing, numeral 101 indicates a main antenna consisting, for example, of an array antenna which is composed of a plurality of radiation elements linearly arranged at equal intervals; numeral 102 indicates a sub-antenna spaced apart from the main antenna 101 along the X-direction, which is the direction of the beam width to be compressed; numeral 103 indicates a transmission circuit, where transmission power to be supplied to the main antenna 101 is obtained; and numeral 104 indicates a multiplication circuit for multiplying the received signals of the main antenna 101 and the received signals of the sub-antenna 102. In the example shown, the direction of the beam axis is perpendicular to the X-Y plane, that is, the plane of the drawing.
In the above-described antenna device, signals received by the antennas 101 and 102 are supplied in phase to the multiplication circuit 104 to be subjected to multiplication processing, whereby the directional characteristic of the main antenna 101 which is, for example, as shown in FIG. 2A, and the directional characteristic of the sub-antenna 102 as shown in FIG. 2B are multiplied, thereby obtaining a synthetic directional characteristic as indicated by the solid line in FIG. 2C, in which the beam width has been compressed from .theta.w to .theta.wc.
When applied to radar, the above-described conventional beam compression method for antenna patterns provides an effect equivalent to beam compression. However, there is a problem that, due to the grating lobe of the sub-antenna 102, if scattering objects which are of the same magnitude in terms of radio wave exist at different positions, a difference in magnitude occurs in the synthetic reception pattern formed through multiplication, that is, a distortion is generated in the synthetic reception pattern.